getting_started: building: fix inaccuracies

The building and running section is out of date, as Windows and macOS
commands are available now.

It also has some accuracies:

- the following section describes how to build hello_world, not how to
  make a new application based on it

- zephyr-env scripts do more than just set project-specific
  environment variables (they modify PATH on Unix, and may run
  arbitrary RC scripts on all platforms)

Fix these issues.

Signed-off-by: Marti Bolivar <[email protected]>

Author: Marti Bolivar

doc/getting_started/getting_started.rst

@@ -70,27 +70,25 @@ Use the following procedures to create a new development environment.
 Building and Running an Application
 ***********************************
 
-Using the 'Hello World' sample application as a base model, the following
-section will describe the pieces necessary for creating a Zephyr application.
+Next, build a sample Zephyr application. You can then run it either in
+emulation or using POSIX APIs available on your host.
 
-The processes to build and run a Zephyr application are the same across
-operating systems. Nevertheless, the commands needed do differ from one OS to
-the next. The following sections contain the commands used in a Linux
-development environment. If you are using macOS please use the appropriate
-commands for your OS.
+If your board is supported by Zephyr (see :ref:`boards` for a list),
+consult its documentation for flashing and running instructions.
 
 Building a Sample Application
 =============================
 
-To build an example application follow these steps:
+Follow these steps to build the :ref:`hello_world` sample application
+provided with Zephyr.
 
 #. Navigate to the main project directory:
 
    .. code-block:: console
 
       cd zephyr
 
-#. Set the project environment variables:
+#. Set up your build environment:
 
    .. code-block:: console